Inflatable packers have been in use in the oilfield for many years. These packers include an inflatable element which expands under the application of fluid pressure into contact with the surrounding casing or tubular to effectively seal it off. Downhole conditions can change with regard to temperature. Downhole pressures can also fluctuate due to changes in the formation pressure or injection pressures applied in the annular space above the inflated element. The pressure and/or temperature fluctuations can be quite large. If the temperature of the element increases, the inflate pressure tends to increase. Conversely, if the temperature of the element decreases, the inflate pressure tends to decrease. If these fluctuations are large enough, an element rupture can occur. Alternatively, the element can release from the casing or tubular because of insufficient internal pressures. Temperature changes are frequently accompanied by applied pressure fluctuations. A cold fluid injected into the well or a zone that is shut off can cause the pressure and temperature effects on the inflated element described above. Experience shows that there are very few instances where a temperature change occurs without an accompanying pressure change in one direction or the other.
Compensation devices have been attempted in the past. One example is PCT application WO 98/36152 assigned to Tech Line Oil Tools A.S. In this design, a single floating piston, having two discrete piston areas with an atmospheric chamber in between, is employed. The purpose of this compensation device is to maintain the inflate pressure at a certain ratio above the well pressure, either above or below the element. This design, however, does not accommodate the discrete responses which occur due to pressure and temperature changes which occur contemporaneously. The compensator described by Tech Line is located below the element and attempts to inflate the element by way of compensation, depending on whether a cool-down or heat-up downhole is anticipated. In other words, the specific phenomenon must be anticipated before the tool is run in the wellbore so that the compensating piston will be in the appropriate position after inflation of the element. If cool-down is anticipated, the compensating piston of this design is completely stroked so that upon cool-down, the compensating piston can move uphole toward the element to maintain the internal pressure. Conversely, the compensating piston is not stroked at all if a heat-up is anticipated. In that manner, when the heat-up occurs, downhole movement of the compensating piston can occur to its opposing travel stop to avoid pressure build-up under the element in response to the surrounding heat-up.
However, where the compensator is below the elements as in the Tech Line design, and cool-down is expected, cold fluid is generally being injected from the surface. In these situations, the inject pressure is applied to the element, followed by subsequent cooling of the element. The inject pressure causes the element pressure to increase, and as the element cools, the inject pressure keeps the inflate pressure elevated and renders the compensator ineffective. This is because the compensator is placed in an initial fully stroked position, and while cool-down would bring it back toward the element, the applied inject pressure overcomes the cool-down effect and keeps the compensating piston bottomed against its travel stop, making the compensation system ineffective. This combination of forces causes the element to deform at the wall where the inject pressure is applied and substantially increases the risk of failure due to the possibility of kinking ribs which can cut the wall of the inflatable element.
Again, in the Tech Line design where the element temperature is expected to increase, an accompanying inflation pressure above the element results in fluid being squeezed out of the element so as to drive the compensating piston down. This occurs because due to the anticipated temperature increase, the compensating piston by design is against its travel stop closest to the element when the element is inflated. In that manner, the Tech Line compensator can compensate for temperature increases as the compensating piston moves away from the inflated element. However, temperature increases, coupled with applied pressures outside the element, add together to bring the compensating piston to its downward travel stop position, once again risking severe deformation and damage to the element.
What is needed is a compensating device that is fully functional for temperature increases or decreases which, at the same time, has the ability to respond to applied increases or decreases in pressure from above or below the element. One of the objects of the present invention is to isolate pressure effects, leaving the compensating device the ability to be fully responsive to increases or decreases in temperature, independent of fluctuations in pressures above or below the inflated element. Those and other advantages of the present invention will be more apparent to those skilled in the art by a review of the description of the preferred embodiment below.